What you describe sounds like powdery mildew, a fungus that attacks all kinds of plants. It is most frequently found in summer and fall, when humidity is high, daytime temperatures are warm and nighttime temperatures are cool. It rarely occurs during hot, dry weather. Once your plant has powdery mildew, it's difficult to control. To stop the spread, you can prune away the affected fronds. There are no chemical controls for palms. Make sure your palm has adequate sunshine and good air circulation all around. Best wishes with your landscape!